Terahertz radiation refers to electromagnetic waves sent at the terahertz range between 300 gigahertz (3×1011 Hz) and 3 terahertz (3×1012 Hz), corresponding to the submillimeter wavelength range between 1 millimeter, in the high-frequency edge of the microwave band, and 100 micrometers, long-wavelength edge of far-infrared light. Terahertz radiation is non-ionizing submillimeter microwave radiation and shares with microwaves the capability to penetrate a wide variety of non-conducting materials. For example, terahertz radiation can pass through clothing, paper, cardboard, wood, masonry, plastic, ceramics, fog and clouds, but not metal or water. Terahertz radiation is emitted as black body radiation from an object with a temperature approximately equal to or greater than ten Kelvin. Possible sources of employable terahertz radiation include a gyrotron, backward wave oscillator, quantum cascade laser, the free electron laser, and synchrotron light sources.